With classical antenna structures, a certain physical volume is required to produce a resonant antenna structure at a particular radio frequency for a specific bandwidth. Much work has been done over time to develop techniques that effectively reduce the antenna size while maintaining performance. As the physical size of an antenna is reduced, the peak gain decreases and the beam width of the radiation pattern increases, thus resulting in a wide beam width low directivity antenna. It tends to be more difficult to control the radiation pattern characteristics of electrically small antennas.
A common antenna type is a microstrip antenna, which is a low profile planar antenna element that can be placed above and close to a ground plane. The ground plane is integral to the antenna and can be on the order of a wavelength for proper operation. As the ground plane increases in size the front-to-back ratio of the radiation pattern increases, resulting in a more optimized antenna when radiation in the forward sector is desired. The increase in ground plane size, however, can be a negative attribute when overall antenna size and cost are considered. The microstrip antenna can be designed with a smaller ground plane at the expense of front-to-back ratio.
Additionally, frequency de-tuning can occur when a microstrip antenna with an undersized ground plane is placed on a larger ground plane. The frequency response of the antenna may shift because of the larger ground plane provides increased structure for coupling. This de-tuning is a common problem due to the desire to design a single microstrip antenna that can be used for multiple applications where there are different ground planes or mounting structures used in the multiple applications.